1. Field of the Invention
The present invention pertains to a circuit arrangement for the cancellation of echo signals comprised of a send path carrying a send signal; a receiving path carrying a received signal; an adaptive filter; an analog first adder; a first digital/analog converter; a transmission unit; and a two line wire attached to the transmission unit, wherein over the two line wire at least the send signal, the received signal and echo signal portions are transmitted, with the send path and the received path being connected with the transmission unit, with the send signal being transmitted to the adaptive filter for the estimation of the echo signal and the estimated digital echo signal being transmitted over the first digital/analog converter to the first analog adder, with the first analog adder being located in the received path, for the reduction of the echo signal portion in the received signal.
2. Discussion of the Background of the Invention and Material Information
Copper lines or wires continue to serve as the main transmission medium of the transfer of data between a subscriber connection and the communications office of a public communication network. A subscriber connection must, on one side, be economically advantageous for the subscriber, yet on the other hand, it is desirable to realize the highest possible data transmission rate. Via the replacement of copper wires with glass fiber cables, the noted second condition could be largely fulfilled, however, this would be coupled with substantially increased costs, since the existing detailed distribution net would have to be completely replaced. For this very reason, the development of a fast digital fully duplex data transmission line, via copper wires, has already been suggested repeatedly. An overview of this suggested data transmission, denoted as HDSL (High Bit Rate Digital Subscriber Line) was, for example, published in an article by J. W. Lechleider, entitled "High Bit Rate Digital Subscriber Lines: A Review of HDSL Progress" published in the IEEE Journal on Sel. Areas in Com., 9(6); pages 769-784, August 1991.
With HDSL data transmission, data is transmitted in both directions via a copper cable. A so-called transmission unit, also denominated as terminating unit, controls the directional separation of the signals to be sent and to be received. Since this directional separation, particularly due to the change in characteristics of the lines, cannot be perfectly realized, the signal received, by a distant subscriber, is superimposed with an echo signal (also denominated as error signal) of the nearby subscriber. The echo signal can be about 30 to 40 dB stronger than the actual useful signal. In order to achieve, at the receiver, the required bit error rate of about 10.sup.-7, an adaptive compensation of the echo is undertaken, in that, via the use of an adaptive filter, the echo signal is estimated and subtracted from the received signal. For such an adaptive filter, for example, FIR (Finite Impulse Response) filters are utilized, which can have 90 to 100 coefficients (Taps) and as a result thereof require a correspondingly large basic circuit block. In order to reduce the circuit expenditure, European Patent Application EP-0 384 490 discloses an echo compensator with an adaptive filter that is comprised of an FIR (Finite Impulse Response) and an IIR (Infinite Impulse Response) filter, in a tandem or cascade connection. Therewith, a reduction of the circuit expenditure for the adaptive filter is achieved, however with this adaptive filter, non-linearities in the send path cannot be compensated.
In addition, European Patent Application EP-0 464 500 discloses an echo compensator in which the estimated echo signal is totally compensated in the analog portion. As a result thereof, even though the circuit expenditure for both the analog/digital converter and the succeeding adder in the received path is reduced, there remains the substantial requirements for the linearity of the digital/analog converters utilized for the transformation of the estimated echo signal. Thus, the reduction of the circuit expenditure for the analog/digital converter is largely compensated for by the utilization of the now required digital/analog converter.
The present invention thus has the task or object to produce a circuit arrangement for compensation of error signals with which the circuit expenditure is both further reduced and the estimation of the echo signal is increased.